Sound recording system



y 30, 1944- c. M. SINNETT 2,349,888

SOUND RECORDING SYSTEM Filed Oct. 21 1942 T q' GAIN a coNTRoL a [J I PRE A REcoRoER To RECORDER I 1 AMPLIFIER I AMPLIFIER MIcRoPHoNE- MONITOR /1 I CONTROL I SUPER-AUDIBLE OSCILLATOR 7 8 e) Tic-1.5.

AUDIO AMPLIFIER TO FREQUENCY POWER EEL I AMPLIFIER VARIABLE AMPLIFIER TO F'LTER GAIN' S S L 10 13 /TO GAIN CONTROL ELECTRODE FILTER GAIN CONTROL FOR DIscRIMINAToR- VOLTAGE HAS A CONTROL POLARITY TO 11/,W5QUENCY RECT'F'ER REvERsE MONITQR CONTRQL 14 TO A.F.

CONTROLLED RECORDER To ORDER SOURCE AMPLIFIER AMPLIFIER suPER-AuoIsLE REcTIFIER U21 OSC'LLATOR /5 I .L 22 F 8 a .4. Lemma? REACTANCE INVENTOR TUBE CHESTER M. SINNETT ATTORNEY Patented May 1944 SOUND nnconnmc s s'rm Chester M. Sinnett, Westmont, N. 1.; assignor to Radio Corporation of America. a corporation of Delaware p Application October 21, 1942, Serial No. 462,831

2 Claims. (cl. ire-100.4)

My present invention relates to sound recording, and more particularly to monitoring of the recording process.

One of the main objects of my present invention is to provide a method of recording sound waves wherein a control tone is embodied in the sound record, and which control tone provides compensation during reproduction for changes in volume effected during the recording step.

Another important object of this invention is to provide a method of recording sound waves wherein a superaudible oscillator has its oscillations recorded on the record at the time of recording the audible frequencies, and the superaudible oscillatim frequency being dependent upon adjustments made to a monitoring de vice.

Another important object of my invention is to provide a method ofreproducing sound records wherein the record embodies a superaudible control tone for compensating for volume changes during recording; the superaudible-c'ontrol tone being utilized during reproduction of the sound record for varying the gain of the reproducing system in such a manner as to provide the aforesaid compensation Yet another object of the invention is to provide a method of automatic compressiorr'of the dynamic range during sound recording, and a superaudible control tone being employed to function as a reference frequency during reproduction whereby restoration of the original volume is effected,

r Still other objects of my invention are to improve generally the efliciency and reliability of sound recordingprocesses, and more especiallyv .'to provide recording and reproducing system wherein sound waves may be faithfully recorded and reproduced. I i

The novel features which I believe to be characteristic of my invention are set forth I with 1 particularity in the appended claimsjthe invention itself, however, as to both its organization 'and method of operation will best be understood device 4 is to permit the operator in charge of the recording process to cut ,down passages ures.

system employed for reproducing sound records .made by the method of Fig. 1,

, vention.

Those skilled in the art are fully acquainted with the circuit details of the various networks which are to be described in the following fig- It is, therefore,. not necessary to show such circuit details, and it is to be clearly understood that the following description isaddressed to those skilled in the art of recording and reproducing sound records. In Fig. 1 the numeral l designates a source of audio frequency currents. Such a source may be a microphone, the detector of a radio receiver, the electrical pickup of a record reproducer, or any other type of device providing audio'frequency currents. The

audio frequency'currents are passed through a pre-amplifier 2 which may consist of one or more stages of audio frequency amplification.

The amplified audio frequency currents may then be transmitted to a stage of audio frequency amplification 3 provided with a. gain control mechanism. The numeral 4 designates an arrow. The latter is to be understood as schematically representing a gain control device.

Such a gain control device, for example, could be a potentiometer for varying the control grid bias employed as a monitoring element at stage 3. The purpose of the gain control, or monitor,

which are too.loud,or to boost in volume those passages which are too soft. The functions of the monitor device are too well known to need further description. The monitored output of stage 3 is transmitted to a recorder amplifier 5. The output of mplifier 5 is applied to any customary and well knownrecord-cutting instrumentality. Since there are many well known cutting devices, and diflerent.modes of cutting the. sound track on the record, it is not neces- ',sary to'describe such devices in detail. It is to be clearly understood, however, that theoutput of amplifier 5 may be applied to afilm, tape or other recording means, if desired.

In addition to recording the monitored sound waves, according to my invention there is provide ed a reference, or compensation, wave on the record which has a frequency dependent in value upon 'the adjustments made to the gain control device 4 The reference wave is of superaudible 2 v aseasea frequency, and is generated by a superaudible oscillator 6. The oscillator may be of the well known R-C' type. Normal, or mean, frequency of the oscillator B is designated by the symbol Fo- Since the oscillator may be of any well known form, itis schematically represented. The oscillator tunable tank circuit is designated by numerals "i and. .8. These numerals denote the' usual coil and variable condenser 8 ,yvhich provide the tuning circuit of the oscillator. Of 10 course, in an R-C oscillator the value of a resister is usually varied to change frequency.

The value of Fe can be chosen to be at any frequency above the range of audibility; say, for example, 17 kilocycles (kc.)

Merely by way of illustration, a dotted line is shown as connecting the gain control device 3 and the variable condenser 8. This dotted line is designated Monitor control. It is to be understood that the monitor control is any mechanical 20 coupling which varies condenser 8, or a resistance in case of an R-C oscillator, in accordance with adjustments of gain control device 4. Thus, ad-

iustments of device t with respect to a predetermined normal amplification value is accompanied by predetermined variations in frequency of oscillator 6 with respect to the normal value Fe.

It is to be understood that the amplitude of the superaudible oscillations will be constant at all.

times.

In Fig. 2 there is shown in a purely qualitative 3 manner, the relation between variations in gain of device 4 and the frequency of oscillator B. It.

will be noted that for the frequency Fe the gain of network 3 is at a mean, or center, value. If

whose frequency variation is a direct; function of the monitoring variations. It is to be understood that the'sense of frequency variation may be the reverse of that shown in Fig, 2. Furthermore, the variations of oscillator frequency may be performed by devices otherthan changes'in the ca-' p'acity of the tank circuits 1-8.

In order to reproduce a sound record which embodies the frequency-variable superaudible control tone there is employed a system as schematically shown in Fig. 3. Here the sound record and'the record pickup device are not shown, since they are well' known tothose skilled in the art.

Let it be assumed that the output of the pickup device is applied to a pro-amplifier 9. The amplified energy is composed of audible frequencies 7 vide changes in gain of an amplifier in the audible frequency path. In this way. volume changes are" returned to their original value as exist at the- .and ,the superaudible control tone frequency.

- Hence,,th'e audio frequency filter' network I0 is provided solely to pass audible-frequencies, while the filter network ll passes solely the superaudible control frequency.

The audio frequency energy, output of filter network I0 is applied to an amplifier I! which has a variable gain. The outputofamplifier I2 is applied to a power amplifier l3, and any desired type of sound reproducer device, such as aloud 7o speaker, may be connected to the output terminals of power amplifier l-3. Theenergy at the output terminals of filter ll of superaudible increased above the value Fe. producing system of Fig. 3 the gain control voltcontrol voltage whose polarity and magnitude are respective functions of the direction and extent of frequency deviation of oscillator 6 from the value F0, there is employed a discriminator-rectifier network M. This network may be of any well known type employed as detectors of frequency modulation receivers, or as a discriminator of an automatic frequency control system of a superheterodyne receiver.

Usually the discriminator-rectifier M comprises a pair of rectifier circuits whose rectifiers are connected to have their rectified voltages in polarity opposition. The input circuits of the rectifiers are oppositely mistuned by equal frequency values from a predetermined mean frequency value. In the present case the latter frequency value may be Fe. It will, therefore, be

- seen that as the frequency of the'oscillator 6 varies above or below Fe, there will be produced at the outputterminals of network l4 in Fig. 3 an output control voltage which is positive or negative. .The polarity will depend on the direction of frequency deviationof the control tone, and the magnitude of the'control voltage will depend upon the amount of frequency deviation of the oscillator. v

The control voltage is applied to a gain control electrode of amplifier l2. This manner of gain control is very well known. The gain control voltage will be applied to the amplifier l2 in a sense such as to compensate for the monitor control. Thus, if the monitor control had been adjusted to increase the gain above normal gain, the superaudible oscillation frequency would be.

Hence, in the reage produced by the increase of control tone frequency above Fe would be a negative voltage.

This negative voltage would be applied to the gain control electrode of amplifier l2, and would trol' tone for record reproduction by means of which manual changes involume during recording can be compensated for during reproduction.

. Generally, I record a s'uperaudible frequency on the record at the time of recording the audible frequency, and the superaudible frequency is varied in accordance with variations of the monitor control. The control tone frequency variation By passing the control tone frequency variation,

through a discriminator-rectifiercircuit a come pensation voltage is secured, and is used to prosource I.

If automatic compression is employed during sound recording, then the change in bias of the compressor network can be used automatically ,to change the frequency of. the superaudible oscillator. 'In that case when the record is reproduced the original volume range can be restored.

In Fig. 4 I have shown a system for securing an automatic compression. Here amplifier 20 is schematically-represented as a controlled amplifier. Let it be assumed that this amplifier has a control electrode thereof subject to bias control by the direct current voltage produced across the dance of the reactane tubehas an inductive, or

output load resistor 2| of a rectifier 22. A portion of the audio input to amplifier is applied to rectifier 22. The voltage produced across output resistor 2! will then vary in magnitude in accordance with the modulation currents fed to amplifier 20.

The control voltage is applied in a negative, or gain reducing, sense to the gain control electrode of'amplifier 2|]. increases, representative of increased volumes,

Hence, as the audio amplitudephase shifter element.

there will be applied increased negative voltage .7

to the amplifier 20. This reduces the dynamic range, and effects volume compression. The output of amplifier 20 may be applied to recorder amplifier 23, and the output of the amplifier 23 may then be recorded as explained above. In-

stead of utilizing a mechanical coupling to the superaudible frequency oscillator 6, there is employed an automatic control over the frequency of the tank circuit 1-8 of the oscillator. For this. purpose there may be utilized any well known type of reactance tube network 30.. Thegain of the reactance tube is varied by means of the connection 3| to the negative end of rectifier output resistor 2 I. The function of the reactance tube is to simulate a reactive effect across the tank circuit 1-8 thereby causing F0 to change in accordance with the degree of compression. An R-C type oscillator may be advantageously employed in this" case. i

Those skilled in the art of frequency modulation transmission are fully aware of the manner" of constructing a reactance tube circuit so as to provide a reactance eifect across an oscillator tank circuit. For example, the control grid to cathode impedance of a tube maybe connected across the tank circuit, while the control voltage connection 3| is made to the control grid of the tube. In this way the control grid to cathode,

capacity of a reactance tube is thrown across the tank circuit, and themagnltude of the capacity effected is a function of the voltage across re-.

sistor 2 l. Alternatively, the cathode to plate impedance of an electron discharge tube may be connected across tank circuit |8. A phase shifter element may then be employed to apply to the control grid of the reactance tube an alternating voltage which is in phase quadrature with the alternating voltage across tank circuit 1-8. In this case, the plate to cathode impecapacitative, effect dependent on the sign of the Here, again, the frequency of network 1-8 may vary with respect to a mean frequency Fe depending upon the variations of the voltage across resistor 2| with respect to a mean value chosen to represent a normal gain of amplifier 20. Of course, the reproducing process will follow that described in Fig. 3.

While I have indicated and described several systems for carrying my invention into effect, it will .be apparent to one skilled in thevart that my invention is by no means limited to the particular organizations shown and described, but

that many modificationsmay be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is: 1. In a system for recording sound waves, means for amplifying audio frequency currents derived from the sound waves, means for rectifying the currents to provide a control voltage, means to control the gain of the amplifying means with said control voltage, means to generate superaudible control oscillations, an electronic reactance device electrically associated with the generating means for control of the frequency of said superaudible oscillations, means responsive directly to said control voltage to vary the magnitudeof the reactive effect of said electronic device whereby said superaudible frequency is varied, and means for recording both said amplified audio currents and said superaudible oscillations. I

2. A method of recording sound waves while compressing the dynamic range thereof which includes amplifying audio frequency currents derived from the sound waves, deriving a control voltage from the currents controlling the amplification with said voltage in a manner which is inverse to the amplitude changes of the audio frequency currents, generating superaudible con trol oscillations, providing an electronic reactance to control the frequency of said superaudible oscillations, varying the magnitude of said electronic reactance with said. control voltage there- .by to vary the frequency of the oscillations in accordance with control of said amplification, and recording both said controlled audio frequency currents and control oscillations.

CHESTER M. BINNETT. 

